is a file called meta.inf a mandatory component, but one which ADE & calibre viewer can live without ?

It's not a file, it's a directory (folder), and it's "META-INF" (case-sensitive, and with a hyphen). It's mandatory, because it must contain a file named "container.xml", which defines where the main OPF file is located. It is this OPF file that defines all the other ePub contents and structure. Maybe ADE and calibre have some way of finding the OPF file on their own, but without "META-INF" the ePub is definitely invalid.

It's not a file, it's a directory (folder), and it's "META-INF" (case-sensitive, and with a hyphen). It's mandatory, because it must contain a file named "container.xml", which defines where the main OPF file is located. It is this OPF file that defines all the other ePub contents and structure. Maybe ADE and calibre have some way of finding the OPF file on their own, but without "META-INF" the ePub is definitely invalid.

I wimped out & did an epub to epub convert with calibre, It is OK now.

but out of curiosity I opened what is now the original_epub file in winrar. & I see all of that "missing" stuff - the META-INF folder, and inside that the container.xml.. so sigil's error message is misleading.

I cannot post the file for copyright reasons but if anyone wants to did deeper juse PM me with an email to send it to

I have seen books where there were deliberate errors in the opf with file extensions. Somehow the book worked fine on most readers, but upon opening in Sigil not everything was loaded. The reason is logical, Sigil seems to use the opf to load all the files. So, if a file has a different name, it is not loaded into Sigil.
Perhaps this is also such a case where a deliberate error/loophole is exploited. Have you checked the original file with EpubCheck?

that is pointing to an .opf file which DOES exist on that path; but sigil's error message implies it's not even getting that far as it's complaining about the containing META-INF folder ?

it's probably not worth taking this further, as it's a "found " book about climate change - I also see archive created by freejZIP.url and an itunesartwork file so I am thinking it's been messed with before I found it